Null Values:

Null values denotes missing data in a table or a database. If a column does not have any NOT NULL constraint then for that column there can be NULL values. By default a column can have NULL values.

We cannot use comparison operators like =, >, < with null values. We can use IS NULL and IS NOT NULL operator to check whether there is a null value or not.

Creating a table with no null values in it:

CREATE TABLE Employees(
   EMPID   INT           NOT NULL,
   NAME VARCHAR (20)     NOT NULL,
   AGE  INT              NOT NULL,
   CITY  CHAR (25)       NOT NULL
   );

In the above query NOT NULL keyword denotes that we have to provide some value for that column . We cannot left it blank, we must have to provide an explicit or a default value.

IS NULL Operator Example:

SELECT Name, City  FROM Employees WHERE City IS NULL;

The above query will return those Employee names and cities whose City has NULL values.

IS NOT NULL Operator Example:

SELECT Name, City  FROM Employees WHERE City IS NOT NULL;

The above query will return those Employee names and cities where City column has no null values in it .

 DELETE FROM Employees WHERE City IS  NULL;

The above query will delete those employee records whose city column contain NULL value.